The disclosure relates to wireless communications systems, and, more particularly, to a wireless LAN that includes at an access point a continually varying antenna array as a technique for mitigating the deleterious effects of multipath signal propagation.
1. Field of the Invention
The invention relates to wireless communications systems, and, more particularly, to a wireless LAN that includes at an access point a continually varying antenna array as a technique for mitigating the deleterious effects of multipath signal propagation.
2. Description of the Related Art
A wireless local area network (LAN) provides a flexible data communication system that may be implemented as an extension to, or as an alternative for, a wired LAN. Wireless LANs transmit and receive data using radio frequency (RF) communications techniques to thereby minimize the need for wired connections. In this manner, wireless LANs combine data connectivity with user mobility.
Wireless LANs have gained strong popularity in a number of vertical markets, including the health-care, retail, manufacturing, warehousing, and academia. These and other industries have profited from the productivity gains of using hand-held terminals and notebook computers to transmit real-time information to centralized hosts for processing. Today wireless LANs are becoming more widely recognized as a general-purpose connectivity alternative for use by a broad range of business customers. Observers have predicted a sixfold expansion of the worldwide wireless LAN market by yearend 2000, reaching more than $2 billion in revenues. The widespread reliance on networking in business and the meteoric growth of the Internet and online services are strong testimonies to the benefits of shared data and shared resources. Wireless LANs enable users to access shared information without the need to establish a hard-wired connection. Network managers have the option to create or augment networks without installing or relocating wires. Wireless LANs offer productivity, convenience, and cost advantages over traditional wired networks. Those advantages largely derive from speed, flexibility and simplicity of installation, reduced cost of ownership, and scalability.
For a thorough discussion of wireless LAN technology, see Jim Greer, Wireless LANs: Implementing Interoperable Networks, Macmillan Technical Publishing (1999), hereby incorporated by reference. In general, the implementation of wireless LANs may be based on one or more of a wide range of technologies, including:
Narrowband Technology. Narrowband wireless systems transmit and receive data or information on a specific radio frequency or within a specific narrow band of frequencies. Narrowband RF techniques strive to minimize the bandwidth necessary to transmit information. Undesirable crosstalk between communications channels is avoided by carefully coordinating different users on different channel frequencies. From an implementation perspective, a salient drawback of narrowband technology is that, in general, the end-user must obtain an FCC license for each site where the technology is employed.
Spread Spectrum Technology. Wireless LAN systems predominately use spread-spectrum technology, a wideband RF technique developed by the military for use in reliable, secure, mission-critical communication systems. Spread-spectrum techniques offer enhanced reliability, integrity, and security, at the expense of increased bandwidth. In other words, greater bandwidth is required than in the case of narrowband transmission. However, the tradeoff produces a signal that is, in effect, more robust and thus easier to detect, provided that the receiver is informed with the parameters of the spread-spectrum signal that is transmitted. If a receiver is not tuned to the correct frequency, a spread-spectrum signal appears as background noise. There are two fundamental types of spread spectrum technology: frequency hopping and direct sequence.
Freguency-Hopping Spread Spectrum Technology. Frequency-hopping spread-spectrum (FHSS) uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise.
Direct-Sequence Spread Spectrum Technology. Direct-sequence spread-spectrum (DSSS) generates a redundant bit pattern for each data bit that is transmitted. The bit pattern is called a chip, or chipping code. The longer the chip, the greater the probability that the original data can be recovered and, concomitantly, the greater the amount of the bandwidth required. Even if one or more bits in the chip are dropped in transmission, statistical techniques embedded at the receiver recover the original data without the need for retransmission. To an unintended receiver, DSSS appears as low-power, wideband noise and is rejected (ignored) by most narrowband receivers.
Infrared Technology. Infrared (IR) represents a third available technology, albeit little used in commercial wireless LANs. IR systems use very high frequencies, immediately below visible light in the electromagnetic spectrum, to carry data. As is the case with light, IR cannot penetrate opaque objects and is, therefore, either directed (line-of-sight) or diffuse technology. Inexpensive directed systems provide very limited range (three feet) and typically are used for personal area networks, but are occasionally used in specific wireless LAN applications. High performance directed IR is impracticable for mobile users and is therefore used only to implement fixed sub-networks. Diffuse (or reflective) IR wireless LAN systems do not require a line-of-sight transmission path, but cells are limited to individual rooms.
In a typical wireless LAN configuration, a transmitter/receiver (transceiver) device, called an access point or, alternatively, a hub, connects to the wired network from a fixed location using standard cabling. At a minimum, the access point receives, buffers, and transmits data between the wireless LAN and the wired network infrastructure. A single access point can support a small group of users and can function within a range of less than one hundred to several hundred feet. The access point, or the antenna attached to the access point, is usually elevated, but may be mounted essentially anywhere that is practicable, as long as the desired transmission coverage is obtained.
End users access the wireless LAN through wireless LAN adapters. Wireless LAN adapters are implemented as PCMCIA cards in notebook or palmtop computers, or as cards in desktop computers, and may be integrated within hand-held computers. Wireless LAN adapters provide an interface between the client network operating system (NOS) and the transmission medium via an antenna. The nature of the wireless connection is transparent to the NOS.
Wireless LANs can range from simple to complex in both design and operation. At its most basic, two PCs equipped with wireless adapter cards can establish an independent network whenever they are within range of one another. Such a network is generally referred to a peer-to-peer network. Ad hoc peer-to-peer xe2x80x9cnetworksxe2x80x9d require no administration or preconfiguration. In this case, each client will have access only to the resources of the other client, but not to a network server or host computer.
Installing an access point can extend the range of an ad hoc network, effectively doubling the range at which the devices can communicate. Because the access point is connected to the wired LAN, each client is afforded access to server resources as well as to other clients. Each access point has the capacity to accommodate many clients, the specific number of clients depending on the number and nature of the transmissions involved. Many applications exist in which a single access point services from 15-50 client devices.
Access points have a finite range, on the order of 500 feet indoors and 1000 feet outdoors. In a very large facility, such as a warehouse or a college campus, more than one access point may be indicated. Access point positioning is accomplished by means of a site survey. The objective is to blanket the intended coverage area with overlapping coverage cells so that clients are free range throughout the area without losing network contact. The ability of clients to move seamlessly among a cluster of access points is called roaming. Access points often are designed to hand the client off from one to another in a way that is invisible to the client, thereby ensuring uninterrupted connectivity.
Although, as may be gleamed from the above, wireless LANs offer significant operational advantages, wireless LAN technology is challenged by a number of phenomena. In particular, high-speed wireless LAN transceivers, such as those intended to be compliant with the IEEE 802.11(b) standard, require consistent strong signal in order to maintain high data throughput. In an office environment, the RF signal traveling between a client device (such as a notebook computer) and an access point (base station) will likely reflect off many objects and surfaces, including walls, office furniture, and inhabitants, in route to the receiving antenna. Due to the density of obstructions in an office environment, it is likely that a signal will reach the receiving antenna through multiple paths. Because of the resulting difference in path length, disparate signals may arrive at the receiving antenna with randomly variant phase relationships. That is, different versions of the same signal will exhibit correspondingly different phase shifts in transmission between the access point and the portable client. This can result in a phenomenon known as multipath fading, or multipath distortion, which is primarily manifested as a time-varying signal amplitude at a receiver, in this instance, at a portable client. In fact, the IEEE 802.11(b) standard contemplates the adjustment of transmitted data rate in response to variations in received signal strength indication (RSSI), such as may result from multipath fading.
In an office environment with a stationary access point, there will exist areas of weak or null signal. These areas of signal nulls commonly result from multipath fading phenomena, in which randomly phased signals travel different paths from the access point and effectively tend to cancel one another at the client. They are, accordingly, sometimes colloquially referred to as xe2x80x9cfade bubblesxe2x80x9d. Fade bubbles predictably increase in size with distance from the access point. If the transmitter (access point) and receiver (client computers) are both stationary, as in the situation, for example, where a notebook computer is placed on a table in a conference room and is communicating with a stationary access point in the building, a condition may arise in which the signal strength at the receiver is inadequate to enable transmission of data at the specified maximum rate, or at all, until the user relocates the receiver (computer) to a different location.
Even in situation where both the receiving device and transmitting device are stationary, the bubbles of signal minima drift unpredictably if inhabitants or structures in the environment are continually or randomly in motion. This effect can fortuitously, but randomly, improve the throughput of a device that is sitting in a fade bubble as described above.
A method commonly employed as a response to multipath distortion involves the use of at least two antennas, with physical separation and/or RF isolation due to cross polarization. The antennas are then said to be spatially diverse, and the technique is commonly referred to as a diversity antenna system. The intended effect is that because the transmitted signal emanates from two antennas, it will travel along two different paths and, presumably, by subjected to different degrees of multipath fading. In this case, the receiver measures signal strength from both antennas and selects the antenna when the RSSI or signal-to-noise ratio (SNR) falls below a threshold. This method is effective when the separation between antennas is sufficient so that at least one antenna will likely be outside the fade bubble, thereby obviating the need to relocate the receiver. However, the implementation of diversity antennas in this manner increases the cost and complexity to the notebook computer design. Furthermore, the effectiveness of diversity antennas remains limited when the size of the fade bubbles become large with respect to the antenna separation.
Accordingly, what is desired is an approach to the mitigation of multipath fading phenomena in wireless LAN environments. A preferred approach will tend to minimize the cost and complexity of the design, manufacture and operation of wireless LAN client devices, such as notebook computers, PDAs and the like. In addition, it is desired that the approach be effective in circumstances where the client device is deployed in a substantially fixed, or perhaps narrowly circumscribed, location, such as in a conference room.
In accordance with one aspect of the disclosure, an acceptable solution is susceptible of implementation in a variety of ways. One approach is to use a single transmitting antenna at the access point and physically translate or rotate the antenna continually. Typically access points in use today have dual xc2xd-wave dipole antennas connected through a customer-accessible coaxial connector. Generally the access point will switch the receiver between the two antennas to maximize received signal strength. Generally, only one antenna will be used for transmitting. With this configuration, the transmitting antenna could be rotated 360 degrees by a motor or could reciprocate continually, in the manner of a windshield wiper. An advantage of this scheme is that it may be made available as a customer-installable option.
Although the above approach is viable and should result in good performance, it does require moving parts and therefore may result in the generation of audible and electrical noise, and may also be susceptible to mechanical wear and tear. Another approach would be to attach an array of antennas to the access point and electrically switch between the antennas periodically. If the antennas have different polarization and radiation pattern characteristics, the desired effect of moving the fade bubbles can be accomplished with only a small spacing between the antennas. This suggests that the device could remain quite small.
The above and other objects, advantages and capabilities are achieved in one aspect of the disclosure by a wireless communications system, such as a wireless LAN, that comprises a plurality of client devices. In one embodiment, the client devices are portable computers that include a transceiver for transmitting and receiving a wireless communication signal. The wireless communications system also comprises an access point (alternatively referred to as a xe2x80x9chubxe2x80x9d) that similarly incorporates a transceiver that is coupled to an antenna array. An antenna controller is coupled to the antenna array and operates to cause a substantially continual variation in the radiation pattern of the antenna array during periods when the access point is transmitting. Continual variation of the radiation pattern of the transmitting access point antenna mitigates the effects of multipath fading by assuring that a stationary client will not be permanently positioned in a signal bubble area, that is, in an area of signal minima.
Another aspect of the disclosure is embodied in a method of mitigating the effects of multipath signal propagation in a wireless LAN. The method is characterized by substantially continually varying the radiation pattern of an antenna system in the course of the transmission of a wireless signal from an access point to a client. The disclosure comprehends numerous techniques for varying the antenna radiation pattern, including, but not limited to, varying the physical position or orientation of the antenna system, switching between antennas of two or more different types, varying the length of one or more antennas in the antenna system, and varying the gain and/or phases of signals applied to antennas in the antenna system.
In another aspect of the disclosure, a computer system comprises at least one portable computer, a network server, and an access point that is coupled to the network server via a wired communications link and that is coupled to the portable computer via a wireless communications link. An antennas system, including one or more antennas, is coupled to the access point, and an antenna controller is coupled to the antenna system. The antenna controller operates to cause the radiation pattern of the antenna system to continually vary during the transmission of a wireless signal from the access point to the portable computer, and, in so doing, mitigates multipath signal propagation effects.
In a further aspect, the disclosure may be exploited in a wireless LAN as a method of mitigating the effects of multipath signal propagation between an access point and a receiving client. In accordance with the method, a wireless signal is transmitted from the access point to the receiving client and, contemporaneously, the radiation pattern of an antenna system is continually varied.
As an additional ramification of the disclosure, an antenna system constituent in a wireless LAN is operable to continually vary the radiation pattern imparted to a wireless signal that is caused to propagate from an access point to a client. The antenna system comprises an antenna array coupled to the access point. An antenna controller is coupled to the antenna array so as to continually vary the radiation pattern of the antenna so that even a substantially stationary client is assured to avoid permanent placement in a position of a signal null that results from multipath propagation.
In a still further embodiment, the disclosure is implemented in a computer system that includes a client device coupled to a source of networked resources. A communications device is coupled to the client device via a wireless first communications link and is coupled to the source of networked resources through a second, likely wired, communications link. An antenna system is coupled to the communications device in a manner that enables the communications device to transmit signals to, and to receive signals from, the client device. The antenna system is coupled to control means for varying the radiation pattern of the antenna system as signal is transmitted from the communications device to the client device.